1. Field of the Invention
The present invention relates to an antenna duplexer, more specifically, the antenna duplexer to be used for a microwave band communication equipment or the like.
2. Description of the Related Art
For example, there is a portable telephone system having a plurality of passing bands of a transmitting circuit and a receiving circuit such as NTACS-CDMA. In the case of NTACS-CDMA, the frequency is allotted to 887-901 MHz and 915-925 MHz for the transmission side, and 832-846 MHz and 860-870 MHz for the reception side, having two kinds of passing bands, respectively. Thus, the transmission frequency bandwidth to be required for the transmitting circuit of the antenna duplexer to be used for NTACS-CDMA is 887-925 MHz while the reception frequency bandwidth to be required for the receiving circuit is 832-870 MHz, and both the transmitting circuit and the receiving circuit are required to have the wide passing band of 38 MHz. On the other hand, the separation to be secured to separate the transmission frequency bandwidth from the reception frequency bandwidth is 17 MHz, and the separation between the two becomes extremely small.
The antenna duplexer synthesizes the phase of the transmitting circuit with the phase of the receiving circuit. In the case of NTACS-CDMA, the transmitting circuit is set to be of high impedance (open) at the reception frequency bandwidth of 832-870 MHz, and the receiving circuit is set to be of high impedance (open) at the transmission frequency bandwidth of 887-925 MHz to ideally synthesize the phase of the transmitting circuit with the phase of the receiving circuit.
FIG. 11 is an example of the circuit of a conventional antenna duplexer 81. In FIG. 11, Tx denotes a transmitting terminal, Rx denotes a receiving terminal, ANT denotes an antenna terminal, 82-85 denote resonators of a transmitting circuit 100, C31-C34 denote coupling capacitors to determine the magnitude of the attenuation in the blocking bandwidth, C35-C39 denote capacitors, L31-L34 denote coupling coils, 86-90 denote resonators of a receiving circuit 101, and C40-C45 denote coupling capacitors.
In the conventional antenna duplexer 81, however, the transmission frequency bandwidth and the reception frequency bandwidth are wide, respective in the case of NTACS-CDMA, and the separation between the two is extremely small, and it is practically difficult to set the transmitting circuit 100 to be of high impedance in the reception frequency bandwidth and to set the receiving circuit 101 to be of high impedance in the transmission frequency bandwidth.
FIG. 12 is a Smith chart of the antenna duplexer 81. In FIG. 12, P1, P2, P3 and P4 respectively indicates the impedance at the frequency of 832 MHz, 870 MHz, 887 MHz, and 925 MHz of the receiving circuit 101 viewed from a branch point A in FIG. 11. The frequency 832 MHz is the frequency at a low frequency side end part of the passing band of the receiving circuit 101 while the frequency 870 MHz is the frequency at a high frequency side end part of the passing band of the receiving circuit 101. The frequency 887 MHz is the frequency at a low frequency side end part of the passing band of the transmitting circuit 100 while the frequency 925 MHz is the frequency at a high frequency side end part of the passing band of the transmitting circuit 100. FIG. 12 shows that the impedance of the receiving circuit 101 at P3 of the frequency of 887 MHz is low, and the insertion loss of the antenna duplexer 81 in transmission is increased. To cope with the problem, Q.sub.0 is increased by increasing the size of the dielectric resonator of the transmitting circuit and the receiving circuit in order to obtain the steep attenuation curve, but there raises another problem that the size of the antenna duplexer 81 is increased.
As a method for miniaturizing the antenna duplexer, a proposal is made that the dielectric resonator is used in the transmitting circuit and the surface acoustic wave filter element is used in the receiving circuit. (For example, refer to Japanese Unexamined Patent Publication No. 5-95204.) However, it is difficult to set the transmitting circuit to be of high impedance at the reception frequency bandwidth or to set the receiving circuit to be of high impedance at the transmission frequency bandwidth though the antenna duplexer can be miniaturized by using the surface acoustic wave filter element, and the insertion loss in the transmission and reception can not be improved. It is more rather superior in the characteristic aspect to compose the transmitting circuit and the receiving circuit of the dielectric resonator.
Also, as the method to set the transmitting circuit to be of high impedance at the reception frequency bandwidth and to set the receiving circuit to be of high impedance at the transmission frequency bandwidth, an antenna duplexer 121 illustrated in FIG. 13 capable of switching two kinds of the passing bands of a transmitting circuit 130 and a receiving circuit 131 is proposed. In FIG. 13, Tx denotes a transmitting terminal, Rx denotes a receiving terminal, ANT denotes an antenna terminal, CONT denotes a voltage control terminal, 122 and 123 denote resonators of the transmitting circuit 130, 124-127 denote resonators of the receiving circuit 131, L35 and L44 denote coupling coils, C50 and C51 denote coupling capacitors to determine the magnitude of the attenuation of the blocking region, C52 and C53 denote capacitors, C54-C59 denote frequency variable bandwidth capacitors, D11-D16 denote PIN diodes, L36-L41 denote choke coils. R11, R12 and C60, C61 denote resistors and capacitors for supplying the control voltage, respectively, L42, L43 and C62 denote coils and a capacitor to constitute the phase shifter, respectively, C63-C65 denote coupling capacitors, and C66 and C67 denote a multi-pass capacitors to polarize the receiving circuit 131. The transmitting circuit 130 constitutes a variable bandwidth blocking circuit while the receiving circuit 131 constitutes the variable band passing circuit.
The antenna duplexer 121 can apparently set the transmission frequency bandwidth and the reception frequency bandwidth to be small, and to increase the separation between the two. However, there is a problem that the antenna duplexer 121 is not suitable for miniaturization because it requires one of the PIN diodes D11-D16 and one of the choke coils L36-L41 for each of the resonators 122-127. In addition, degradation of the resonance system Q.sub.0 (Q.sub.0 is the Q at the center frequency) is not avoided because the PIN diodes D11-D16 and the capacitors C54-C59 are connected to a large number of resonators 122-127 in parallel. In particular, the insertion loss of the receiving circuit 131 to constitute the band passing circuit is dependent on the resonance system Q.sub.0, and electric degradation of the receiving circuit 131 is remarkable.